1. Field of the Invention
The present invention relates, in general, to cooling electric circuitry and to controlling the operating temperatures of electric modules such as power modules, and, more particularly, to power module assemblies configured to have low thermal resistances including a reduced number and thickness of thermal resistant or insulator layers, such as those in a semiconductor substrate, between a power module (or other electric component or element that generates heat) and a coolant and/or a heat sink.
2. Relevant Background
There continues to be a growing demand for power semiconductor or integrated circuit (IC) modules and other electric elements with higher power density, higher reliability, and improved cost effectiveness. Miniature power and electric modules are used extensively in a range of applications including automobiles, hard drives, data storage devices, and nearly any electronic product. These power and electric modules often operate at high voltage and current which causes them to generate a large amount of heat which causes the modules to become hot. For proper operation, the power and electric modules or elements need to be maintained below predefined operating temperatures to avoid performance degradation or failure, such as 125° C. for many power modules but the desired operating temperature may vary with the particular application. However, the goals of miniaturization and increased performance run are in conflict with effective thermal management because higher power demands result in greater heat generation while increased module density reduces the size of cooling surfaces. As a result, cooling problems plague all electronic systems and are particularly troubling in electronic systems utilizing IC power modules, such as Insulated-Gate Bipolar Transistors (IGBTs), power metal-oxide-semiconductor field-effect transistors (MOSFETs), and the like.
Electronic component designers have a number of options for attempting to dissipate or remove heat generated by an electronic circuit, such as power module, but none have adequately met the demands for higher performance with reduced size. Designers may use convection to remove heat by transferring heat away from the electric components with air flow. This is useful for small portable devices such as cell phones but is not as effective for power modules that are tightly spaced in enclosed spaces such as are present in automobiles and other products. In these cases, convection is supplemented or replaced by conduction in which heat is wicked or transferred away from the hot or higher temperature electronic circuit or power module to cool or lower temperature of components that contact or abut the electronic circuit or the substrate upon which the circuit is mounted. Typically, a coolant may be used to remove heat from the lower temperature components, e.g., water or another fluid may be pumped over a portion of components contacting the base of the electronic circuit to maintain a lower temperature. In such conductive designs, increased heat transfer has typically been obtained by increasing the rate at which heat is transferred from the electronic circuit, as measured by the heat transfer coefficient of the electronic circuit or power module assembly. Unfortunately, such techniques of increasing the heat transfer coefficient have not been able to keep pace with the demand for increase module density and have caused many modules to be designed to operate at levels below performance capacity, e.g., at reduced power levels.
A specific example of an electronic system that must be designed for operating within an acceptable temperature range and therefore, for heat dissipation, is an IC power module assembly such as an IGBT device. FIGS. 1–3 illustrate a conventional design for a power module assembly 100. As shown, the power module assembly 100 includes a heat sink 110 made up of a spreader plate 112 and a number of channel walls (or heat transfer fins) 114. A cooling medium or coolant, such as water, is pumped through the walls 114 contacting the walls 114 and spreader plate 112 and removes heat from the heat sink 110 as the inlet coolant, CIN, is at a lower temperature than the outlet coolant, COUT. The assembly 100 also includes a circuit substrate 120 that is mounted on the spreader plate 112 of the heat sink 110 and a power or circuit module 130 that is mounted on the circuit substrate 120. The power module 130 is shown in FIG. 3 to include a circuit layer, e.g., a silicon die with an IGBT, power MOSFET, or the like, 136 and a connection or joint layer 132, such as solder or other joining materials, for joining or mounting the circuit layer to the circuit substrate 120. Heat that is generated in the power module 130 is conducted through the circuit substrate 120 and heat sink 110 to the flowing coolant.
The thermal resistance of the power module assembly 100 depends mainly on the thermal resistance of the spreader plate 112 and circuit substrate 120. The circuit substrate 120 may take a number of forms, but in the case of an IGBT power module assembly 100, often take the form of a Direct Bonded Copper (DBC) substrate that is thermally bonded to the heat sink 110. In this example, as shown in FIG. 3, the circuit substrate 120 is thermally bonded to the spreader plate 112 with a thermal bonding layer 122 that typically would be thermal grease or paste. The circuit substrate 120 includes a ceramic isolation layer 126, such as layer of AIN or other ceramic material, sandwiched between two conductive layers, such as layers of copper or the like, 124, 128. As can be appreciated, maintaining the temperature of the power module 130 requires heat to be conducted through the bonding layer 132 of the power module 130, through the circuit substrate 120 including the thermal bonding layer 122, and through the spreader plate 112.
Attempts to improve the heat exchange characteristics of power module assemblies, such as assembly 100, have not been entirely effective requiring power (and corresponding heat) produced by the power module to be limited. Most conventional techniques have concentrated on increasing the heat transfer rate or coefficient on the fluid side on surface 114, such as by employing micro channels, by using extensions from the spreader plate (such as the channel walls or fins 114 in heat sink 110 of FIGS. 1 and 2), or by using other techniques to improve heat transfer effectiveness of the power module assembly.
Hence, there remains a need for improved designs for dissipating heat generated by electronic circuits, such as power modules. Preferably, such improved designs would be compatible with existing module designs and manufacturing techniques and would significantly reduce the thermal resistance between the component generating the heat and the heat sink and/or coolant.